In fabricating hollow bodies such as, for example, large 55 gallon drums, it has been the practice to extrude a hollow tubular shaped, thermoplastic preform or parison from an extrusion machine into an open blow molding apparatus. The walls of the blow molding apparatus are thereafter closed and air is blown into the preform to cause it to expand and move outwardly against the surrounding walls of the blow molding apparatus. Often in forming such a hollow body it is desirable to provide annular protrusions around the circumference of the wall of the tubular member. These protrusions may be external or internal of the wall of the hollow body. Internal protrusions are suitable where simple reinforcement is desired. Externally disposed protrusions can provide rolling rings for the container or chime members, such as L-shaped chimes, to be used in connection with handling the hollow body.
Where an annular protrusion is to be formed the wall of the molding apparatus is constructed to provide one or more annular recesses into which the plastic material of the preform is blown during the blow molding operation. These annular recesses extend circumferentially around the inner wall surface of the molding apparatus and are defined by a fixed mold surface, facing axially of the preform, and an axially movable slide member. After the blow molding operation is completed the slide member is moved axially into the recess to form the annular protrusion to the desired configuration, the protrusion being formed in the recess and its shape being dictated by the shape of the recess.
If these protrusions are formed from the unreinforced wall material of the preform which has penetrated into the annular recess of the molding apparatus, it may happen that there is not enough of this material to form a proper protrusion with evenly distributed material. This is so because when the preform is blown out into the annular recess, it is stretched and thus thinned out. Then, as the slide member of the molding apparatus is moved axially against the material in the recess, the material is folded and welded while in a still hot formable state. This basic procedure is disclosed in the British Patent Specification No. 882,789. If there is an insufficient supply of material, weak points in the form of notches can be created which increase the risk of tearing in the event of swell pressure due to the toppling of a filled hollow body.
For this reason, the walls of the preform material have been made thicker in the area of the annular protrusion so that, during the subsequent forming process, sufficient upsetting material is available. The increase in wall thickness in the preform at the axial position where the protrusion is to be formed is created by changing the gap cross-section of the extrusion nozzel during the extrusion of the preform. As a result of this procedure, however, the increased thickness extends in a broad band over an axial area of the preform.
Control of the wall thickness by changing the cross-section of the extrusion nozzle is described, among other places, in the British Patent Specification No. 1,493,453 and U.S. Pat. No. 3,275,726. Also, the Journal "Kunststoffe" ("Plastics") of May 1978, page 314 ff, describes and shows, FIG. 1, an L-Ring Barrel of 220 1 capacity. FIG. 1 is a sectional view with the measurement values of the thickness distributions axially of the casing. The description explains that the two circumferential, massive L-shaped rolling and lifting hoops have been generated integrally from the preform by means of axial upsetting of the preform. The typical process described above is employed to provide the additional quantity of plastic for forming the outwardly extending protrusion. But, as also described above, control of the extrusion orifice results in a broad-band axial thickening of the preform. A significant disadvantage of this procedure is that more material than necessary must be used for forming the protrusion. This is so because additional material making up the thickened wall normally tends to sag as the preform is extruded and thus shift out of the location where the protrusion is to be formed. This downward migration is due to the weight of the material making up the additional thickness. To overcome this, the thickening is effected over the wide band area.
U.S. Pat. No. 4,170,623 purports to show the formation of an annular thickening only in the immediate area of the annular recess of the blowing mold. This thickened area is, as stated in the patent, formed during the extrusion of the preform in accordance with well-known techniques. According to the drawings of this patent the thickened portion is narrowly restricted axially of the preform and maintained in its alignment with the annular recess in the molding apparatus until it is blown into the recess. Since, however, the thickened wall of the preform is formed during the extrusion of the preform by controlling the nozzel size, the drawings of this patent do not accurately show the true behavior of this thickened portion. As indicated above, a sharply delimited narrow annular thickening in the preform will migrate downwardly from the horizontal plane where it is initially formed during the extrusion process. Also with the thickness of the preform being greater adjacent the mold dividing seams, the downward migration of the thickened area will vary about the circumference of the preform. More particularly, the thickened portion adjacent the mold dividing seams being heavier will move downwardly further than the thickened portion disposed at 90.degree. from the mold dividing seams. Therefore, if the thickened annular portion of the preform penetrates at all into the radial recess of the molding apparatus during blow molding, it does so only in partial regions as measured circumferentially of the preform. Accordingly, no satisfactory ring formation or protrusion is effected.